Microencapsulation is the envelopment of small, solid particles, liquid droplets, or gas bubbles with a coating, usually a continuous coating. Many terms are used to describe the contents of a microcapsule such as active agent, active core, core material, fill, internal phase, nucleus, and payload. The coating material used to form the outer surface of the microcapsule is called a coating membrane, shell, or wall. It may be an organic polymer hydrocolloid, sugar, wax, metal, or inorganic oxide. Microcapsules usually fall in the size range of between 1 and 2000 microns, although smaller and larger sizes are known.
In interfacial polymerization reactions, the fill is typically a liquid rather than a solid. Interfacial polymerization involves the reaction of various monomers at the interface between two immiscible liquid phases to form a film of polymer that encapsulates the disperse phase. The monomers diffuse together and rapidly polymerize at the interface of the two phases to form a thin coating. The degree of polymerization can be controlled by the reactivity of the monomers chosen, their concentration, the composition of either phase vehicle, and by the temperature of the system.
Microcapsules produced through interfacial polymerization having shell walls composed of polyamides, polyureas, polyurethanes, and polyesters are known; see U.S. Pat. Nos. 3,516,941, 3,860,565, 4,056,610, and 4,756,906. In some instances the shell walls of these conventional microcapsules are very porous and consequently relinquish their fill too rapidly for some applications. Therefore, the microcapsules may have to be post-crosslinked with such crosslinking agents as polyfunctional aziridines. The crosslinking provides shell walls with greater structural integrity and reduced porosity. Of course, an obvious disadvantage to post-crosslinking of curing is that it adds another step the microcapsule production process.